The applicants co-pending application, publication number WO 88/02875, describes a Sagnac interferometer in which the symmetry of the two counterpropagating directions along the coupling means is broken so as to obtain a relative phase shift in the counterpropagating portions of the input signal. This can be achieved by, for example, providing a coupling ratio of other than 50:50 and an optically non-linear optical fibre waveguide constituting the coupling means. In this case the intensities of the signal portions coupled into the ends of the waveguide are not equal. If input signals are of sufficient intensity to produce self-phase modulation of the optical portions as they propagate around the optical fibre loop, the signal portions propagating in opposite directions around the waveguide will experience different refractive indices due to the Kerr effect. This results in the signals experiencing different phase shifts so that when the signals return back to the coupling means they have an intensity dependent relative phase shift.
The intensity dependence of the relative phase shift results in a device whose output at an input port is an oscillatory function of the intensity of the input signal. Any signal exiting the second input port (ie the port to which the input signal is not coupled) is said to be "transmitted" by the interferometer. This property can be used in a variety of applications including logic elements, optical amplifiers, optical switches and the like although complete switching is not obtained due to the non 50:50 splitting.
A disadvantage of this configuration is that the small Kerr effect coefficient of currently available optical fibre materials limits its use since a large optical power loop-length product is required to produce the necessary phase shifts.
A known approach to obviating this disadvantage is to incorporate an asymmetrically located optical amplifier in the loop with the Sagnac loop having a 50:50 coupler as disclosed in an article entitled "Nonlinear Amplifying Loop Mirror" by M E Fermann, F H Haberl, M Hofer and H Hochreiter, Optics Letters vol 15 no 13, Jul. 1, 1990. The position of the amplifier provides that the counterpropagating portions of an input signal have different intensities for a large proportion of their transit around the loop. If the amplified portions are of sufficient intensity to activate the non-linear regime of the optical fibre there will be an intensity dependent relative phase shift between the counterpropagating portions resulting in the required intensity dependent switching.
In the prior art embodiment just described the fibre loop was 306 m in order to achieve a sufficiently large intensity-loop length product.